Most spacecraft require attitude information relative to a reference frame in order to perform motion control or for mission operations. For earth satellites, an Earth Centered Inertial (ECI) frame is often preferred because it simplifies references for operations personnel.
Typically a spinning spacecraft, such as a satellite, may make a series of control actions to modify attitude and orbit. Preceding each control action is an attitude determination phase. Accurate attitude determination is critical to mininizing the number and extent of each control action. Preceding each attitude determination phase may be a nutation dampening phase to increase the accuracy of the attitude determination.
Many systems and methods are available for determining the attitude of spinning spacecraft. Such a method is described in U.S. Pat. No. 5,020,744 issued to Schwarzschild. The initial spin axis attitude is established by a ground station. An additional sensor, such as a sun sensor, is used to determine the attitude within the plane of rotation. Angular rate information, obtained from gyroscopes, can than be used to propagate positional information. Because gyroscopes drift, frequent recalculation of the attitude by the ground station is required.
Several difficulties exist with current methods for determining the attitude of a spinning spacecraft. A first difficulty is hat current methods allow continuous on-board attitude updates from a sun sensor only once the attitude is determined with respect to a sun normal reference frame. If the attitude is determined with respect to an ECI reference frame, the attitude must be computed on the ground and transmitted to the spacecraft with time critical commanding. Further, once the attitude in the ECI frame is uploaded to the spacecraft, the attitude can only be propagated by the data from gyroscopes. Output from other sensors, such as sun sensors, and the spin axis data cannot be used on-board by tho spacecraft to update the attitude.